X-ray fluorescence analysis is a common and widely used tool in analysing the contents of certain elements and/or compounds in given samples. As a first example we will consider the task of analysing the sulphur content of liquid hydrocarbons, such as petrol (gasoline), diesel oil and other liquid fuels. Environmental considerations have promoted the enactment of relatively tight limits for allowable sulphur content in liquid fuels. A measurement arrangement should be able to reliably measure concentrations in the order of only few ppm (parts per million).
FIG. 1 illustrates schematically a prior art arrangement for measuring the sulphur content of a liquid fuel sample through X-ray fluorescence analysis. The sample 101 is poured into container, which in this arrangement is a cup 102 supported in a holder 103. The bottom of the cup 102 comprises a window 104 for X-rays to pass through. An X-ray tube 105 or some other suitable source of X-rays is used to irradiate the sample 101 through the window 104. Fluorescent X-ray quanta from the sample 101 are collected and counted in a detector 106. The whole arrangement is located in an enclosure 107 that comprises a valve 108 for flushing the measurement arrangement with a suitable protective gas, such as helium or nitrogen.
Sulphur has a fluorescent emission line at approximately 2.3 keV (kiloelectronvolt). Very close to it is a 3 keV fluorescent line of argon. Pure atmospheric air contains around one percent argon, which means that accurate results cannot be obtained using a proportional counter if the space 109 between the X-ray source 105, the window 104 and the detector 106 is filled with air. Flushing the measurement arrangement with hydrogen, helium or nitrogen has been regarded as mandatory to keep air (and thus argon) from interfering with the measurement.
FIG. 2 illustrates a second prior art example, which in this case corresponds to measuring the concentration of certain elements in metal alloys. A measurement head is housed in a gastight enclosure 201 and comprises an X-ray source 202, X-ray directing means 203 and a detector 204. A contact surface (here the top surface) of the enclosure 201 is designed to allow placing the measurement head very close to a metallic sample. A window 205 in said contact surface is permeable to soft X-rays and allows excitation radiation from the X-ray source 202 to pass through to the sample, and fluorescent radiation induced in the sample to enter the measurement head and hit the detector 204. A flushing arrangement 206 comprises means for flushing the inside of the enclosure with a gas, typically hydrogen or helium. The open space 109 within the enclosure 201 between the X-ray source 202, the window 205 and the detector 204 must not contain any substance that would interfere with the measurement.
Detecting elements from metal alloys with an arrangement like that in FIG. 2 typically involves measuring the fluorescent emission lines of aluminium, magnesium or silicon, with energies of 1.49 keV, 1.25 keV and 1.74 keV respectively. The 3 keV line of argon causes little interference with these measurements, but the absorption of the fluorescent radiation in air becomes a problem—hence the need for gas flushing.
The constant need for flushing is a problem, because flushing gases of the required purity are not cheap, because the required settling time before the actual measurement can begin is relatively long and because the gas containers and tubing tend to make the overall appearance of the apparatus somewhat clumsy.
An obvious alternative for flushing the measurement arrangement with a gas would be to produce a vacuum into the relevant space 109. However, concerning the application shown in FIG. 1, liquid hydrocarbons are highly volatile even at normal atmospheric pressure, and exposing them to a vacuum would cause the whole sample to evaporate very quickly. The relative softness (i.e. the low energy level) of the X-rays involved requires the window 104 to be very thin, and made of a material that does not absorb X-rays of the energy involved to any considerable extent. It is not possible to have a vacuum only in space 109 and to have the sample at normal atmospheric pressure, for example by using the construction of FIG. 2 and replacing the flushing arrangement 206 with a vacuum pump, because none of the known window materials could stand the pressure difference.
Eliminating an empty space between the X-ray source, the window and the detector altogether has not been regarded as a feasible solution either. The soft X-rays involved only penetrate the sample to a depth of a few tens of micrometres. The sample surface area that is irradiated and from which fluorescent quanta are collected must have a reasonable size, at least several square millimetres. Direct propagation of X-rays from the X-ray source to the detector must not be possible. All solid materials that could be used to fill the empty space absorb the X-rays too much. All these boundary conditions have precluded the appearance of measurement arrangements with no empty space 109.
Yet another known drawback of the prior art arrangement is the possible change in measurement geometry, caused by creeping deformation of the window. This is a problem especially in the case of FIG. 1. Being made of a polymer, typically polypropylene, and being constantly exposed to hydrocarbon solvents, the window material is prone to stretching. Even if a careful selection of materials could prevent any damage caused by chemical incompatibility, it is possible that a person conducting the measurement uses a too heavy sample or leaves the sample in the cup for an extensively long time, in which case the sheer mechanical load can cause a permanent deformation of the window 104.